Phenolic resins are widely utilized in numerous industrial areas such as in the preparation of adhesives, coatings, and various molded articles. There are two basic types of phenolic resins which are defined by the type of process utilized to prepare the phenolic resin. The first type of phenolic resin is a resole phenolic resin, referred hereinafter as resole, which is prepared by the reaction of a phenol or substituted phenol with an excess of formaldehyde in the presence of a base catalyst. Resoles are methylol-terminated (--CH.sub.2 OH) and are usually mixtures of monomeric phenols and higher condensation products. Because the methylol phenols are capable of condensing with themselves and with other phenols to form higher molecular weight species, they are typically stored under refrigerated conditions. The resole prepolymers are transformed into three-dimensional, crosslinked, insoluble, and infusible polymers by the application of heat.
The second type of phenolic resin is a novolak phenolic resin, hereinafter referred to as novolak, which is prepared by reacting an equimolar or excess amount of phenol with formaldehyde under acidic conditions. This acid-catalyzed reaction results in the formation of low molecular weight condensation products linked by methylene bridges. Novolak prepolymers are not heat reactive and require the addition of a crosslinker such as formaldehyde or hexamethylenetetramine to form the final cured phenolic resin. Resole compounds can also be utilized as crosslinkers for novolaks, as the methylol groups of the resole will form methylene bridges with the novolak to form the final, cured phenolic resin. Formaldehyde, sometimes generated from self-condensation of the resole, will also act to crosslink the novolak.
As described above, heat-activated resoles tend to be unstable and there is a continuing desire to formulate resoles which have enhanced storage stability. Aqueous solutions of resoles are particularly unstable and the current industrial trend towards the replacement of volatile organic compounds with water creates a substantial technical challenge with respect to obtaining water-based resole compositions with extended shelf-stability.
There have been many previous attempts to improve the stability of resole compositions by chemical modification of the resole. For example, U.S. Pat. Nos. 2,579,329; 2,579,330; 2,598,406; and 2,606,935 describe methylol phenols where the phenol group has been converted to an ether group. The resulting methylol phenol ethers are described as having excellent shelf life (i.e., they can be stored for long periods of time at normal temperatures without danger of condensation or polymerization) while being curable upon the addition of a catalyst and heat.
U.S. Pat. Nos. 4,120,847; 4,122,054; and 4,157,324 describe resole compositions wherein the methylol groups have been converted to methylol ethers. The resoles are described as being storage stable and being curable at elevated temperatures. In addition, in U.S. Pat. Nos. 4,403,066; 4,433,119; and 4,578,448 are disclosed hemiformals of phenolic compositions wherein hemiformal groups are formed at the phenolic hydroxyl and methylol groups of phenol-formaldehyde resins. The compositions are described as being stable at ambient conditions and curable upon the addition of a catalyst and heat. By stable, it is stated that at 25.degree. C. to 30.degree. C. the time required for the viscosity to double is greater than 200 days.
There are several drawbacks associated with utilizing stable resole compositions such as those described above, as curing agents in phenolic resin compositions. The first drawback is that resoles etherified at the phenolic position typically require relatively high temperatures and/or relatively strong catalysts to effect activation and cure. In other cases, unbound alcohol, resulting from hydrolysis of etherified methylol groups, may interfere with the effectiveness of the phenolic resin in its final application. Finally, many of the resole prepolymers stabilized by previously developed methods, such as those stabilized as methylol ethers, can maintain relatively good storage stability as solutions in organic solvents but cannot withstand extended exposure to water as is required in the formulation of water-based phenolic resole compositions.